1. Field of the Invention
The subject matter disclosed generally relates to industrial tables such as optical tables.
2. Background Information
There have been developed various tables for industrial use such as optical bench tables or vibration shaker tables. U.S. Pat. No. 5,154,963 issued to Terry discloses an industrial tabletop that has a top plate, a bottom plate and a honeycomb structure that separates the plates. The honeycomb structure allows for threaded apertures in the top plate. External vibration-sensitive payload devices such as an optical component of an optical system, or a device under test in a shaker system, can be attached to the threaded apertures of the table.
In many experimental research and industrial applications it is desirable to isolate the tabletop from external vibration such as the natural tremor of the building structure. U.S. Pat. No. 6,209,841 issued to Houghton et al. discloses an active isolation module that can be placed between the building floor and a tabletop to dampen tabletop vibration. The isolator includes a vibration sensor and an actuator that are connected to a controller. The sensor senses vibration and provides an output signal to the controller. The controller then processes the output signal and provides a drive signal to excite the actuator and offset the vibration.
The vibration isolators reduce the vibration transmitted to the tabletop from the floor. The tabletop top itself, however, has its own natural frequencies and corresponding flexural vibration modes that can be easily excited by residual vibration coming through the isolators or by other sources such as acoustical excitation, air turbulence and dynamic forces generated by the payload equipment installed on the table. The main flexural vibration modes usually have a global character, which means that an excitation at any point of the tabletop generates a vibration pattern encompassing the whole tabletop surface. Those natural vibrations are very lightly damped and therefore can reach high amplitudes unless special damping means are introduced into the tabletop structure.
Passive dampers of various designs are widely used in construction of optical tables. The “Shock and Vibration Handbook”, ed. By C. M. Harris, 4th edition, 1996; 5th edition, 2001, Ch. 37, provides a survey of the state of the art in this field and a classification of dampers (damping treatments). According to it, the known types of damping treatments include:                Free-layer damping treatments, where the energy is dissipated by means of extensional deformation of a damping layer (made of visco-elastic material) induced by flexural vibration of the base structure.        Constrained-layer damping treatments, where the constraining layer helps induce relatively large shear deformations in the visco-elastic layer in response to flexural vibration of the base structure, thereby providing more effective energy dissipation mechanism.        Integral damping treatments, including use of damped laminated sheets and/or damped joints in the construction assembly.        Tuned dampers, which are essentially mass-spring systems having resonances matched (tuned) to the resonance frequency of the base structure. The application of the tuned damper replaces the resonance peak of the base structure, typically, by two peaks of lesser amplitude.        Damping links, i.e., visco-elastic elements joining tow parts of the structure that experience large relative motion in the process of vibration.        
Some of cited damping techniques have found applications in optical tables. In particular, Newport Corporation (see “The Newport Resource” catalog by Newport Corporation, 2003) uses tuned dampers, constrained layer treatment of work surfaces and integral damping in its optical tabletop designs.
Nevertheless, the growing demand for high precision and high throughput in optoelectronics and semiconductor industries, as well as the needs of modern scientific experimental instruments, require higher damping performance of optical tables than that achieved by the methods and devices known in the state of the art. Active vibration control means are known to be able to achieve superior performance compared to passive control.
It is sometimes desirable to monitor the vibration level on the table. For example, in a precision measurement system the real-time vibration data could qualify or disqualify a certain measurement. In a precision manufacturing system, the real-time vibration data could indicate an increased probability of a particular manufactured item, such as a semiconductor wafer, being defective. Vibration monitoring is also necessary if the tabletop in question is part of a vibration test setup.
The vibration signal may be used merely to indicate increased vibration levels on the platform during certain periods of time. In this case the vibration sensors can be placed at almost any point of the tabletop because of the global character of main vibration modes; the areas near the corners of the tabletop represent a good place for vibration sensors since these areas are responsive to all typical vibration modes of the tabletop top. In other cases, the exact value of vibration input at a critical vibration-sensitive equipment location is of interest. In this situation the sensors should be placed immediately adjacent to the attachment points of the vibration-sensitive equipment.
Deployment of vibration measurement systems, including sensors and cables, on the working surface of the tabletop would detract from the valuable payload space. It may be impossible to place the sensors near the most vibration-sensitive pieces of equipment due to space restrictions. In a production environment it may be impractical due to required set-up time. Therefore, a system monitoring the vibration of the tabletop while leaving its surface clear and accessible to the user would be very desirable.
The essentials of the optical tabletop design are disclosed in the U.S. Pat. No. 4,621,006, entitled “Honeycomb tabletop manufacture and clean-room compatible honeycomb tables” issued to Terry et al. and U.S. Pat. No. 5,500,269, entitled “Honeycomb tabletop manufacture and clean-room compatible honeycomb tables” issued to Terry. Additional details and variations can be found in U.S. Pat. No. 4,645,171, entitled “Honeycomb tabletop” issued to Heide, U.S. Pat. No. 5,061,541, entitled “Honeycomb tables” issued to Gertel, U.S. Pat. No. 5,626,157, entitled “Optical table” issued to Galpin et al. and U.S. Pat. No. 5,962,104, entitled “Optical Table” issued to Gertel et al. For an extensive general description of optical honeycomb tables, reference may be made to the 2000 Vibration Control Catalog and 2002-2003 “The Newport Resource” Catalogs by Newport Corporation. Catalogs of TMC, Kinetic Systems and other manufacturers also contain descriptions of optical tabletop designs. Although the main purpose of an optical tabletop is to provide a stabletop platform for vibration-sensitive equipment, none of the designs described in the aforementioned patents and catalogs include a built-in means for monitoring vibration, or a tabletop that includes active vibration control.